Radio guidance system



July l5, 1952 T. M. FERRILLQJR 2,603,779

. l RADIO GUIDANCEA SYSTEM Filed neg. F27, 1945 l v 2 SHEETS-SHEET 1 .gNvENToR Y Tuo/was M. FERR// ,JR.

' ATTORNEY T. M. FERRILL, JR RADIO GUIDANCE SYSTEM July 15, 1952,l

2' SHEETS-SHEET 2 Filed Deo. 27, 1945 BOVE GOL/RSE o. 25 o BELOW 0N col/RSE col/RSE o 25 0. 5 ,qaovs cou/asf 0.25" o BEL ow o/v coz/RSE col/Rss Patentecl July 15, 1952 ATENT OFFICE RADIO GUIDANCE SYSTEM Thomas M. Ferrill, Jr., Hempstead, N. Y., assignor to The Sperry Corporation, a corporation of Delaware Application December 27, 1945, Serial No. 637,371

The present invention relates to radiant energy systems for the guidance of dirigible craft, and is particularly concerned with methods of and apparatus for indicating the position of a craft relative to a pair of divergent directive energy patterns, which may be oriented in space for partially overlapping directivity.

Radiant energy systems employing a radio transmitter with a directive antenna system have been provided for the guidance of dirigible vehicles such as ships and aircraft. For accuracy in dening a desired course for the dirigible craft, the transmitted energy is often divided between tWo directive radiation patterns or beams aimed in divergent directions, but being of such breadth as to form a region of overlap of energy transmission, such as overlap region containing a Zone of equal eld intensities 'of the two directive patterns. The transmission in the patterns is characterized in a manner which makes it possible to distinguish the reception of energy transmitted in one beam from that transmitted in the other beam, and to determine from Which directive beam the energy of greater amplitude is received. In such systems, a radio receiver is carried by each craft which is to be navigated in reliance upon the spatial distribution of the directive beam energy. The operator in the craft aurally or visually determines the Ycraft position relative' to the directive beam or beams by the use of headphones or by use of a visual indicator suitably coupled to the radio receiver.

In some instances, the transmission in each beam is interrupted at such a series of relatively long intervals that Morse kcode impulses are formed. Such systems have been arranged for interlockingV transmission in the two beams, as with the code elements of the letters A and N, or with a series of Es (Morse dots) transmitted in one beam and a series of Ts (Morse dashes) transmitted in the other beam. The code impulses transmitted in one directivity pattern or beam are timed in exact coincidence with the intervals between the impulses transmitted in the other pattern or beam.

In the use of a dual-beam radio navigation system employing interlocking code identification of the' beams, the operator of vthe craft Wears a pair of headphones connected to the radio receiver, and translates the Morse code received from a selected station to determine his position relative to the equisignal zone defined by the two directive beams. When the craft arrives at the equisignal-Zone, which usually deiines a predetermined pathl for the craftto fol- 3 Claims. (Cl. 343-110) 2 low, the series of code impulses received through one beam are of equal strength to the series of code impulses received through the other beam, and since the impulses of the rst beam are coincident with the intervals between the code impulses of the other beam, the receiver provides a continuous note or tone in the headphones, and no code impulses are heard by the craft operator. The continuous note indicates that the craft is inthe equisignal zone 0f the directivity patterns, and is thus on the predetermined course defined by the directive transmitting system. f

Several attempts have been made to provide visual indicators adapted to supplement or to replace the aural code determination of craft position relative to the A-N 'and E-T systems. Special electromechanical code translators have been devised for this purpose, and attempts have been made to employ non-linear detector and galvanometer apparatus for the purpose of providing visual indication of craft position. The electromechanical code translators proved generally impracticable. rEhe non-linear detector schemes, While able to aord substantially unambiguous indications for great departures from course, Were inherently so nonlinear in instrument deection current relative to departure from course as to render such systems unsuited for craft guidance.V The need-,for visual indication was so strongly felt that the Morse code system of beam identication was in many instances discarded altogether and replaced by visual-indication systems employing modulation of divergent radio beams at different selected audio frequencies. In one system, the beams were modulated at respective frequencies of and 150 cycles per second; in another, they were modulated at 600 and 900 cycles per second.

In one early system employing beams modulated at different frequencies, visual indicators employing a plurality of resonant reeds placed side by side Were provided for indicating to the craft operator the sense and extent of departure from the equisignal course. As these indicators did not prove entirely satisfactory, they were superseded by electrical frequency selective systems. In an indicating arrangement of this type, a plurality of band-pass lters were employed, one for each beam modulation frequency, and each filter was connected to an output rectier. The rectified output voltages were opposed to operate a zero-center galvanometer,

The use of frequency-selective iilters and rectiiiers operating a galvanometer according to the relative strengths of signals received through two beams or directivity patterns of radiant energy modulated at different frequencies has been almost universally adopted for visual-indication radio navigation systems. This indicating arrangement has the disadvantages that the frequencies of modulationY of radio energy transmitted through the two beams must be maintained substantially constant, to avoid variation of attenuation in the frequency-selective filters and consequent apparent shifts of the indicated course. ordinarily afforded by an energy-absorption modulator, so that the beam modulation usually is provided at some sacrifice of the generated power. Moreover, band-pass filters meeting the stringent requirements for reliable performance in radio navigation systems ordinarily are cumbersome and expensive, and theirruse results in appreciable loss of receiver output power.

Various attempts` have been` made to overcome these disadvantages of differential-frequency beam modulation systems by the use of oppositelyphased modulations of theA beams at a single frequency, and phasedetection of the signal produced bydetection :and amplification of the energy transmitted, through the directive radio beams.` The systems which have beenproposed for -oppositely-phased modulations comprehend transmissions of equal-duration impulses alternately in onebeam and the other beam, together with the transmission of a phase-referencesignal to: the craft. .The phase-referencesignal has been` communicated. to the craft by a separate radioV channel. Such opposite-phase modulation systems,A however, have involved such complexity of transmitting equipment and receiving and. indicatingapparatus that they have been generally abandoned, and the systems employing distinctive modulation frequencies have remained in general usage.

An object of the present invention `is to provideA a simple, reliable `and inexpensive system and apparatus for the identiiication of the directive radio beamsand for providing an un,-

ambiguous measure of the position of a craftk relative to a course defined by the beams.

Another object ofA the present invention is to provide an improved system for economical and ecientvisual indication of craft position relative Yto a pair'of divergent radio beams.l

More specifically, it is an object of the present invention to provide a simple radio position indicating system characterized by steady, unambiguous, and substantially linear defiectionr of a visual indicating instrument; by efcient use of generated 4 radio-frequency energy; by freedom from close-tolerance requirements as to` transmitter output modulation frequency; and by simplicity, economy, compactness and light weight of the position detection. components associated with the-receiver.

A further object of -the invention is to provide'apparatus and instrumentalities embodying novel features andr principles, adapted for use in realizing the above objects and alsov adaptedrfor use-in. other fields.

The inventionY in another of its aspects relates to novel features of the instrumentalities describedl herein for achieving the principal objects of vtheinvention and to novel principles employed in thoseinstrumentalities,l whether or not these features and principlesare-used for the said principal objects or in the said field.

Also, the audio-frequency modulation isv According to a major feature of the present invention, radiant energy such as ultra high frequency radio energy is transmitted in one and then the other of two divergently-aimed antenna directivity patterns, the alternation between the patterns being accomplished at a relatively high frequency, and the time duration of the transmission impulses in oney of Athe directivity patterns being appreciably greater thanthe duration of the impulses transmitted in the other directvity pattern. A radio receiver carried by a craft and tuned to the transmitted energy receives a vsubstantially unmodulated signal when the craftvis infthe zone of equal radio-frequency signal intensities-of=the two beams. When the craft is displacedy in a first direction or sense from this zone,the.receiver produces an alternating output voltage characterized by short-duration, fiat-topped, high-potential positive peaks or maxima separated by long-duration, flat-topped, low-potential negative peaks or maxima. When the craft isdisplaced Vin the opposite direction or sense from the equisignal zone, the receiver produces an alternating output voltage characterized by positive` peaks of long duration and. lowpotential, separated by negative peaks of short duration and high potential. Two rectifying circuits are coupledto the receiver for producing twodirect voltages of magnitude varying according to thepmagnitudeof the alternating output voltage of the receiver; anda zero-center galvanometer or otherutilization device isconnected.

tothe-,rectifying circuits for providing an indivoltages. One of the rectifier circuitsisarranged as ak peak-voltmeter circuit, which may include a half-wave rectifier element and a resistancecapacitancestorageandv biasing circuit coupled thereto. Theother rectifier circuit may be similar to the rst, with the half-wave rectifier element connected forresponse to voltage peaks of the polarity opposite that to which the detector element of the first rectifier circuit is responsive; or it may be a type of rectifier circuit independent of asymmetry of positive and negative voltage peaks, e. g., a circuit producing 4a direct output voltage of 'strength dependent directly upon the effectivevalueY of the alternating voltage.

The effective value-of the alternatingvoltage produced by the receiver varies substantially linearly with displacement of the craft in either sense from the equisignal. zone of the two directivity patterns or beams.y The positiveand negative-peak amplitudes of the voltage vary linearly with displacement from the equisignal zone or course, but the positive-peak amplitude is greater than the negative-peak amplitude for a craft displacement in a first sense, and smaller than the negative-peak amplitude, by any equal ratio, forv -a displacement in theopposite directionor sense froml the equisignal zone. Thus, if al zero-center galvanometer or other utilization deviceis connected tocompare thevdirect output voltages of two opposed peak voltmeters, it will be subjected to an actuating current whose direction of flow corresponds to the direction or sense of displacement from course, and whose magnitude varies substantially linearly with the extent of the craft displacement from course.v A corresponding result is obtained if the galvanometer or otherrutilization device is connected between thevoutput terminals of a half-wave peak-voltmeter type rectifying circuit and a rectifier circuit arranged to deliver equalv voltages for equal craft displacements inl opposite directions or l senses, these voltages being exceeded in a predetermined ratio by the directroutput'voltage of the peak-voltmeter rectifying circuit when the craft is displaced in one direction from the equisignal zone, and being in excess of the direct output voltage of the peak-voltmeter rectifying circuit by an equal ratio when the craft is displaced in the opposite direction from the equisignal zone.

The above features of the present invention will be readily understood and other objects will become apparent from the following description of illustrative embodiments of the invention, considered in conjunction with the drawings, where- 1n: i Y v Fig. 1 shows a radio system for vertical guidance of aircraft along a desired instrumentlanding path or course;

Fig. 2 schematically shows a transmission systern adapted for transmitting distinguishable upper and lower glide-path beams;

Fig. 3 illustrates the alternation between Vone and the other of the transmitted beams and the unequal lengths of transmission impulses therein;

Fig. 4 illustrates the composite radio signals received by the radio receiver at various indicated positions of the craft relative to the path-defining beams; p

Fig. 5 illustrates the detected audio-frequency output of a receiver at the various positions corresponding to the received signals shown in Fig. 4; l

Fig. 6 illustrates an instrument landing receiving system including opposed peak-voltmeters coupled to a craft position indicating galvanometer;

Figs. 7 and 8 are graphs illustrating the principles of operation of the present invention;

Fig. 9 illustrates an alternative circuit for the opposed peak voltmeters and indicator;

Fig. 10 illustrates a further modification of the craft position indicating system; and

Fig. 11 illustrates a modification of the present invention employing an asymmetrically-responsive rectifier circuit in opposition to a symmetrically-responsive rectifier circuit. l

In Fig. 1 there isshown in vertical cross-section a pair of radio energy directivity patterns I and II. The energy transmitted in these patterns or beams emanates from a transmitter T employed for defining a radio instrument-landing glide-path for aircraft. The angular widths of the directivity patterns are exaggerated for clarity of illustration. Actually, such directive patterns or beams may be of the order of 2 to 3 in vertical cross-sectional angular extent, the axes i and of such beams ordinarily being symmetrically disposed respectively above and below an axis b of equal beam intensities. This axis b represents the desired line of descent of aircraft to the edge of an airport. Such a line of descent to the airport preferably is inclined at an angle of 2 to 21/2", according to the obstructions in the vicinity lof an airport, and according to the landing characteristics of the aircraft landingin reliance `upon the system.

In accordance with a feature of the present invention, radio-frequency energy from the transmitter T is transmitted alternately in the upper beam I and the lower. beam II, the transmission in the upper' beam constituting a series of impulses each including a train of, radiofrequency oscillations,` and the transmission in the lower beam similarly constituting a seriesV of impulses each including a train of radiofrequency oscillations. The oscillations of the lower-beam impulses are maintained during the interval between two successive impulses of the upper beam. The duration of the upper-beam impulses is purposely made unequal to the duration of the lower-beam impulses, in order that position-indicating circuits involving a peakvoltmeter rectifier arrangement may be employed for indicating the relative` intensities of the radio-frequency oscillations received through the upper beam and the radio-frequency oscillations received through the lower beam. e

Referring now to Fig. 2, the transmitting station T may comprise a radio transmitter III coupled to an upper-beam directive radiator I and a lower-beam directive radiator 2 for generating the upper and lower beams I and II, respectively, the coupling of the transmitter to each. of the directive radiators being accomplishedV through an electromagnetic switch or modulator. The switches or modulatdrs vmay include serrated discs I3 and I4 cooperating respectively with cavity resonators I5 and I6 interposed between transmitter III and the directive radiators I and 2, respectively.l Serrated discs I3 and I4 may be arranged on a common shaft I'I and may be driven by a disc-rotating motor I8. The relative angular positions of the discs I3 and I4 on shaft I1 are so arranged that switch I3, I5 arrests the transmission of radio frequency energy through directive radiator I while switch I 4,v I6 permits the radiation of energy in pattern II through directive radiator 2; and conversely, switch I3, I5 permits the transmission of energy in pattern I by directive radiator I while the switch I4, I6 prevents the transmission of energy through directive radiator 2.

In Fig. 3 are indicated at I and II, respectively, the successive limpulses transmitted in the upper beam and the successive impulses transmitted in the lower beam, the impulses of the two beams being shown in relation to a common time` base or axis of abscissae. Eachv of the switches or modulators I3, I5 and I4, I6 may be arranged for preventing transmission of energy when an enlarged-radius sector of a disc is passing through the cavity resonator. At the instant when the discs I3 and I `4 are in the positions in which they are illustrated Ain Fig. 2, transmission is thus being effected only through the upper beam generating radiator I, as at time t1 in Fig. 3.

As will clearly be seen from an examination of the apparatus shown in Fig. 2, the enlargedradiusV sectors of disc I3 are of relatively great angular extent, providing for relatively long intervals] between the brief successive impulses transmitted in the upper directive pattern or beam I. The enlarged-diameter sectors of disc I4 are of relatively small angular extent, corresponding roughly to the angular extent of the small-radius sectors of disc I3, for providing relatively short intervals .between the long-duration impulses transmitted in the lower beam II of Fig. 1. Modulating and switching arrangements involvingjserrated discs cooperating with cavity resonators in the manner of the units I3, I5 and I4, I6 (Fig. 2) are described in greater detail in U. S. patent application S. N. 444,668, filed May 27, 1942 by D. F. Folland et al., now Patent No. 2,426,992, issued September 9, 1947.

While each of the sector discs I3 and I4 is arranged for the transmission of two impulses per revolutio-n, this'construction is only illustra- 7 tive. Each sector "disc 'may be .arranged foron'e impulse and one interval per revolution, or .may be arranged for the transmission'of Amorethan two impulseslper revolution, as-desired.

In Fig. 4' are. showntime-representations of' Y intervening impulsesor trains of radio-frequency oscillations of relatively lowA intensity. This plot represents-the radio-frequency energy which is received by a radio receiver onboard anaircraftapproaching the transmitter 'I' along-.the linea infFig. 1;'`

' If thefcraft'approaches the` transmitter Talong the desired''instrument landing glidepath 'b`,

which' is 'dened by the Zone of equal radio-frequency intensi-tiesofbeams I and II, the radio-frequency energy received by the radio receive'rA onboard the craftWill appear tobe unmodulated, 'asvrepresented"infFigjl-BL If the craft is below the glide-path, as-for example, if itis travelling along theline c in Fig. 1,'the radio-frequency energy received willl be as represented in Fig. 4 0. The modulation envelopes of the radio frequency signals iii-Figs. 4-A and 4-C are generally similar, except that, there is an eiective 'polarity'reversal relation between them.

Uponldetectionan'd amplification of these signals, there vare produced alternating voltages as illustrated inv Figs. g5-A andrr-C, respectively, the wave forms Aof the alternatingvoltages being similar, but lbeing reversed as to relative polarities. The wave form ink Fig. 5-C consequently appears'as a mirror-image of the wave form in Fig. 5-A. When the craft is on course, no alternating output voltageis` produced, as indicatedV in Fig. 5-B.

In the alternating voltage lwave illustrated in.

Fig. 5-A, the positive peaks or Vmaxima are of appreciably shorter `duration than' the negative peaks or maxima of .the.wave, .and accordingly, the positive peak amplitudeofthis wave isappreciably greater than thenegative peak amplitude. This lcondition correspondswi'th a craft displacement above thecourse `b (Fig. 1),l eg., with a craft position on line ag I l When the craft is displaced. from the course in the Vopposite direction, however, there is `produced a voltage as represented in Fig. 5-C, wherein the negative peak amplitude is appreciably greater than the positive peak amplitude. The amplitude of the shorter-.duration peaks or maxima is higher than that of .the longer-duration peaksY ormaxima of oppositepolarity, in inverse proportion to the ratio of .time durations of the peaks or maxima. sistent with` the requirement that the time integration of .the instantaneous positive values of voltage in each cycle o-f the alternating-voltage Wave must be equal to the time-integration of the negative values of voltage in the cycle.

Referring to Fig. 5 it will be noted that for a condition vof departure from the landing glide path b, the output voltage has a rectangular shape with the peaks thereof provided with attopped configurations.

In Fig. 6 there is shown a radio receiver 2| arranged to supply an alternating output voltage'through a coupling capacitor 22 to a pair of opposed peak voltmeter type rectifying circuits 23 and'24. Peak voltmeter circuit 23 com- This will be readily understood as conlower beams.

prises a rectifier 25 connected inv series with an energy storage circuit yincluding 'a resistor 26 and a capacitor'21 connected in parallel. The peak voltmetercircuit 24 comprises a rectifier 28 connected in series with an energy storage circuit including a resistor 29 'connected in parallel with a capacitor 3 I.

The resistance values of resistors 26 and 29 preferably are equal, and similarly the capacitance values of capacitors 21 and 3| are equal. The relative values of resistance and capacitance in the storage circuits are selectedA for a time constant which is appreciably greater than the period of alternation' between the -upper and e n In `this illustrative circuit arrangementv the peak ,voltmeter circuits 23 and alternating output voltage of'receiverf2l is applied :to each.- The term peak voltmeterl as applied to circuits 23..;and `2li,4 is employed' in reference to .therelati'orvi between direct output voltagevand applied'voltage, the direct output voltages of these circuits being substantially equal tov the 'peak voltage amplitudes of the respective polaritiesto which theyare responsive.

The rectiers25 and 28 are connected in opposite conduction senses, in order that one of the rectiers, e. g., rectifier 25, will .be responsive to positive peaks lof theA output alternating voltage and the other Will beresponsive to the negative peaks of the same voltage. The rectifiers connected in this manner may be said to constitute an' opposed rectifying circuit. A galvanometer 32 is connected-in series With a resistor 33, and the series combination 32, 33 is connected in parallel Withthe opposed peak voltmeter circuits 23 and- 24.' If desired, a microammeter maybe connectedfin series with Veach of resistorsv 26' and 29,'and these galvanometers can be calibrated'for indicating thedirect Avoltages stored 'inv circuits`26, 21 and 29, `3|, these voltages beingsubstantially equal -to the respective peak voltages to .which the-'respective peak voltmeter circuits 23;- 24 are responsive. Such individual peak voltage indicating galvanometers, however, are not essential to the present system,

since galvanometer 32v provides directly a corn-y parison of the opposite-polarityrpeak voltages.

Let it be assumed that the voltage Waveshown in Fig. 5-A represents the time-variations of alternating output potential at receiver output terminal 35 relative ,to receiver output terminal 36. Rectifier 25 .Will permit the flow of unidirectional chargingA currents to the-storage circuits 26, 21 during the positive peaks 31, 38, 39 and 4E of the .wave ,.while-vthe rectifier 28 will permit the oW of unidirectional charging currents during the negative peaks 4|, 42 and 43. As a result of these currents, there'is maintained across storage circuit26, 21 a direct voltage of the polarity indicated in Fig. 6 and of magnitude slightly lower'fthan the positive peak voltage magnitude arbitrarily represented vas substantially-two `volts in Fig.V 5*-A; and similarly, there is maintained across storage circuit 29, 3| .a direct voltage ofthe opposite polarity, and of an appreciably smaller magnitude corresponding to the smaller magnitude of the negative peak amplitude in Fig. 5'A. By virtue of this inequality of the directv voltages maintained across the storage circuits 26, 21 and 29, 3|, the direct currents ow'ingfthrough resistors 26 and 29 will be unequal', the currentthrough resistor 26' being proportionately greater vthan the current through resistor 29.

This condition can exist only if there is a current owing in a further 'path between conductors 45 and 46, the average magnitude of which is equal to the difference of magnitudes of the currents through resistors 26 and 29, and the direction of. which is similar to thedirection of the smaller lof the currents through resistors 26 and.29. This follows from the fact that the energy storing circuits are each comprised of a resistorand capacitor, the latter element forining an effective blocking device for the direct currents produced in the rectifying portions of the circuit. Inasmuch as different voltages'are developed on the condensersY of the energy storing circuits, direct currents of unequal magnitude will be made to flow through the resistors Y 26 and 29, as well as a current having a value equalto the difference of these currents through another. portion of the circuit. Galvanometer 32 in series with resistor 33 provides the path for this difference current. The direction of defiection of the galvanometer pointer vfrom its neutral position indicates the direction of current flow through the path 32, 33 while the magnitude of this deflection indicates the average magnitude of this current flow.

When the instantaneous polarities of the receiver output wave are reversed as in Fig. -C, due to displacement of the craft below the course, the negative peak voltage exceeds the positive peak voltage, and a larger voltage is produced across resistor 29 than that produced across resistor 26. Under these conditions a diierence current must again flow through resistor 33 and galvanometer'32, butin the opposite direction. Accordingly, the direction of current ow through thefgalvanometer 32 is unambiguously indicative of the sense or direction of departure of the craft from the course. When the craft is on course, the. received radio-frequency energyvis unmodulated so that no alternating voltage is produced .atY the receiver output terminals, and accordingly, the pointer of indicator 32 is then neutrallypositioned.

As thejdisplacement ofthe craft `fromthe course increases,ythe average amplitude of the receiver output alternating voltage increases substantially proportionately, so that the extent of the deflection of the galvanometer pointer from its neutral 1 position indicates vsubstantially linearly the extent of the craft departure or displacement from course. Figs. 7 and 8 graphically depict the manner in which'the apparatus in Fig. 6 produces a galvanometer deflection unambiguously and linearly indicating the direction and extent of craft displacement from the cours/e.l Plot 9| represents the variation of positive-'peak amplitude and plot 92 represents the variation of negative-peak amplitude of the receiver output alternating voltage withvariation of craft displacementfromthe equisignal zone or course. These variations are substantially linear with displacement in either direction from the course.V The right-hand" portion of plot 9| has a slope substantially double the slope `of the right-hand part of plot 92, while the left-hand portion of plot 192 has a slope equal to that of the right-handportion of lplot 9|, and substantially double the slope of theleft-hand portion of vplot9l. Y -I The galvanometercurrent-at any displacement from course is proportional to the algebraic difference.. ofnthe positive-peak voltage andthe negative-peak voltage. In Fig. 8, plot 93 represents the variation of the intensity land polarity of current through galvanometer 32 (Fig. 6), and hence the variation of the galvanometer needle deiiection with craft position. Since the positivepeak amplitude and the negative-peak amplitude of the receiver output alternating voltage vary substantially linearly with displacement from course, and the direct voltages stored in circuits 25, 2l and 29, 3| vary accordingly, the current through galvanometer 32, being proportional to the difference of voltages in circuits 2E, 2l and 29, 3|, must vary linearly, in the manner indicated by plot 93 (Fig. 8). A linearly responsive galvanometer 32 is accordingly enabled to provide ideal craft positional indication, such that a :pilot may accurately operate the craft entirely in reliance thereon.

The present invention lends itself readily to'a variety of modifications. Vacuum tube rectiiiers 25' and 26 may be employed in opposed peak-voltmeter-type re'ctifyingv circuits, as illustrated in Fig. 9. The indicating galvanometer 32 may, if preferred, be connected between conductor 46 and the junction of resistors 26 and 29, and 4resistor 33 may then be connected directly between conductors 45 and 46. The essential features of :the operation of the circuit in Fig. 9 correspond generally to those described in detail above for the indicating circuit shown in Fig. 6, the galvanometer 32 again being employed to indicate the sense and extent of the difference of average current intensities through the peak voltmeter storage circuit resistors 26 and 29.

An alternative circuit arrangement providing similar features, but including vacuum-tube amplification, is illustrated` in-Fig. 10. The radio receiver 2| is here shown coupled through a transformer 45| to the grid-cathode circuits of a pair of triodes, which may be portions of a single vacuum tube or may be individual triode vacuum tubes. i'

A first peak voltmeter resistance-capacitance storage circuit including a resistor 52 and a capacitor 53 is connected in the grid circuitof the first triode 54, and a further resistancecapacitance. storage circuit including a resistor 56 and a capacitor 51 is connected in the grid circuit of the other triode 58. The grids of triodes 54 and 5B are supplied in opposite polarities by a center-tapped secondary winding 59 of transformer 5|. The grid-cathode circuit of triode 55 acts as a rectifier and functions as a positivepeak voltmeter in cooperation with capacitor 53 and resistor 52. The grid-cathode circuit of triode -58 similarly functions as a negative-peak voltmeter in cooperation with capacitor 57 and resistor 56.

The anodes of the triodes 54-and 58 may be connected as shown to the positive terminal of a direct voltage source 6|, whose negative terminal is connected to the transformer secondary centertap and to the intermediate tap of a potentiometer 62 connected between the cathodes of the triodes.

' An indicating meter or zero-center galvanometer 32 is connected between the cathodes of the triode Sectio-ns, and is employed for indicating the difference of the voltage drops produced between the intermediate tap of the potentiometer 62 and the ends thereof, due to the difference of cathode currents flowing through triodes 54 and Due tothe rectifier action of the gridfcathode' circuit of triode 54, a direct voltage is developed across capacitor 53 when an alternating output voltage is produced betweenY the output terminals of radioreceiver 2l. A direct voltage is similarly produced across capacitor 51. Each cf these capacitor voltages is so polarized as to maintain the associated grid negatively biased with respect to the corresponding cathode. The magnitudes of the bias voltages across capacitors 53 and 51 are diiferent, however, when the negative-peak amplitude of the receiver output voltage; is diiferent from the positive-peak amplitude thereof. When the craft is above the instrument landing glide path, the negative bias potential developed across capacitor-53 exceeds the negative bias potential developed across capacitor 51; and when the craft is displaced in the opposite direction from the signal Vpath, the'bias potential developed across capacitor 51 ,exceeds that across capacitor 53. The relative cathode currents of the two triode sections arecorrespondingly of different magnitudes, and thus, the galvanometer 32 connected between the cathodes of the triodes 54 and' 58 is deected in sense and extent according to the senseand magnitude of the craft displacement from the course.

In each of the foregoing embodiments of the present invention the craft position indicating galvanometer has been employed for comparing the negative peakvoltage with the positive peak voltage of the asymmetrical alternating voltage wave resulting from reception ofalternate radiofrequency energy impulses of different intensities. Some of the features of the present invention may be realized by comparing a first direct voltage varying as the amplitude of peaks of a selected polarity of the receiver output alternating voltage, With a further direct voltage varying as the average or the maximum of the positive Vand negative peak voltages or as the average effective value of the alternating output voltage.

. In Fig. 1l is shown a radio receiver 2l supplying its alternating output voltage through a coupling capacitor 22 to a positive peak voltmeter 23 and to a rectifier circuit 12 arrangedfor indicating the average eifective value of the alternating voltage wave. Circuit 12 comprises a transformer 13 having a center-tapped secondary winding connected to a pair of rectifier elements-15 and 16. Transformer 13 and rectier elementsz15 and 1B cooperate as a full-wave rectier system, and supply a rectied voltage to a load resistor 11. A resistance-capacitance lter including a series resistor 18 and a capacitor 19 may be connected Y, across resistor 11, and a D.- C. voltmeter 8| may be connected across capacitor 19 for providing a visual indication of the full-wave rectifier output. A second D. C. voltmeter 82 may be connected across the resistance-capacitance storage circuit 26, 21 of peak voltmeter-23, if desired, for providing an indication of the amplitude of output voltage peaks of a chosen polarity, e. g., Aof the positive voltage peaks. The craft position indicating galvanometer 32 and the resistor33 in series therewith maythen be-connected between the positive output terminal 84 of the peak voltmeter 23 and the positive output terminal 85 of voltmeter circuit 12, andthe negativeoutput terminals of circuits 23 and 12 may be connectedftogether, as by a conductor 86.

With this circuit arrangement, galvanometer 32 and series resistor 33`together function-.as a zero-center D. C. voltmeter for indicating the difference of voltages registered by the peak voltmeterA23 and the average effective voltmeter-.Acir- Quit12. A f

Effective voltmeter 12.may be convertedY to a peak voltmeter adapted for control by eitherpositive or negative peaks ofthe alternating output wave, whichever maybe stronger, Vbymaking Vthe resistance value of resistor 18 very small compared to those of resistors 11 and 33, or by eliminating resistor 18. With this arrangement, Where the positive and negative peaks of the alternating voltage are unequal, only that one of the rectiner elements 15 and 16 responsive to the polarity of the higher voltage peaks will-conduct the charging currents necessary to maintain capacitor'19 charged to a direct voltage substantially. equal to the maximum peak amplitudeof the'alternating voltage supplied by receiver 2 I. Y

A voltmeter 12 symmetrically responsive to both positive and negativepeaks ofv alternating voltage,v or responsive vto the average effective value of alternating voltage,' provides voltage l measurementsk independently l'of the polaritiesrof peaks inthe asymmetrical Wave of Aalternating voltage at the output of the receiver. Thus, voltmeter 12 provides equal voltage measurementsat equal displacements of the craft above and below the course b (Fig. l). `The turns ratio of primary and secondary windings oftransformer 13 may be such that the voltage across voltmeter 8| may be made equal to the'meanof thevoltages acrossvoltmeter 82 atfdisplacements of equal angular magnitudes above and below theicourse;

The-principles -of operation of the indicating system shown in Fig.-11 may be-compared to those of Fig. 6 by referenceto the graphs in'Figs. 7 and 8. In Fig. '1, plot '94 represents the variation of VD. C. output voltage;measured by voltmeter 8i as the craft moves froml a position far below the course to a -positionfar abovethe course. Plot-9| in Fig. '7, as hereinbefore pointed out, represents the variation of positive peak voltage amplitude-with the craft `position .as the craft moves throughthe indicated displacement'range.

In thecircuit arrangement'shown inFig. 11,1the craft yposition galvanometer 32 fis .arranged to indicate the variationwith craft. position of the algebraic difference betweenthe direct. voltage across voltmeter 82 (which.variesasfthepositive peak voltage represented by. plotl9 I) andthe direct voltage kEav. across voltmeterl `(varying symmetrically with :displacements above and below course, as illustrated by'plot') l The current I galv. through galvanometeri32 is represented by plot A- (Fig. 8), and, Yfor a given alternating output voltage from receiver 2 l, is of substantially one-half the intensityg'of ,the current Igaiv. owing through the'fgalvanometer 32 in the circuits ofl Figs. 6 and '7:

From the foregoing description and explanation of operation of the circuit arrangement shown in Fig. 11, it will be seen that the principal advantages of the present invention may berealized with a single peak voltmeter rectifying circuit arrangedfor output comparison with a symmertically-responsive voltmeter circuit. Such anarrangement, however, involves slightly greater complexity and slightly less sensitivity than the arrangements employing a pair ofA oppositelysensed peak voltmeters.

.The electrical characteristics of the .capacitors and vresistors used-in .the energy-storage circuits associated with the-rectifier elements are ldetermined according to the frequency of the impulses transmitted.-` For best result sLthe storage capacitorsy mustxbe of'lcapacita-n legvalues inversely pro-i portional to the frequency of the transmission impulses. In order that these componentsy may be of such small capacitance values that they may be made relatively compact, an appreciable impulse modulation frequency, e. g., 180v cycles per second, is selected for the system. This frequency is not critical; a practicable system may be built for any frequencywithin the audible range or the supersonic range of frequencies. Such `high impulse frequencies preclude the aural determination of craft displacement by Morse code translations; but the advantages of compact component parts and steady needle indication in the craft position galvanometer more than offset the elimination of the aural code facilities.

It will be readily apparent that the system above described possesses some similarity to the systems employing equal impulse durations in the two beams and requiring phase-detection receiving circuits. The inequality of the alternate transmission impulses in the two'directive patterns, however, obviates the separate transmission and reception of a phase-reference signal, and makes possible the use of greatly simplified circuits for actuating the position-indicating galvanometer in accordance with the output of the receiver detector circuit. The continuous and efficient use of all of the generated radio-frequency transmitter power is accomplished in this system, as in the equal-impulse phase-detector systems, but this advantage is here enjoyed without the disadvantages of complexity of the phasedetection systems.

The receiving system of the present invention is lighter, simpler, more compact and more economical than the band-pass filter systems heretofore used with distinctive beam modulation frequencies, and moreover, the present invention is extremely tolerant of the modulation frequency. A variation f modulation frequency of 20%, e. g., is tolerated without objectionable results.

In the present invention, since the alternating voltage at the output circuit of the craft guidance radio receiver varies substantially linearly with displacement from course over an appreciable range of craft movement, and since the craft position galvanometer is substantially linearly responsive to the difference of rectified output voltages of two linear rectifying circuits, a substantially linear relation is established between craft displacement from course and craft positional indication, affording ideal information by which to operate the craft.

While the invention has been illustrated as employed for delecting the needle of a positionindicating zero-center galvanometer, it will be readily apparent that the electrical difference signal applied to the galvanometer may be applied to other utilization devices, e. g., to a cra-ft autopilot, for automatically controlling the operation of the craft along the course dened by the radio energy beams.

While the present invention has been illustrated in application to an instrument landing glide path system, it will be readily appreciated that it is equally applicable to an instrument landing localizer system, or to a radio range system for aiding ships or aircraft in navigating along a predetermined course about which the directive energy patterns or beams are symmetrically disposed.

As many changes could be made in the above construction and many apparently Widely different embodiments of this invention could be made Without departing from the scope thereof,

1.4 it is intendedrthat all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. V

What is claimed is:

1. A radio position-defining or guidance system comprising means for transmitting radio-frequency .energy alternately at an audible frequency rate in first and second radiation directivity patterns or beams, the transmission of energy in said rst pattern or beam being characterized by time durations different from the transmission in said second pattern or beam, radio receiving means responsive to the energy transmitted in said two directivity patterns or beams for producing an alternating output voltage When signals of different strengths are re- Ceived through said iirst and second directivity patterns or beams, and a pair of rectifying circuits coupled to said receiving means and each arranged to produce a direct voltage varying according to variation. of said alternating output voltage, one of said rectifying circuits being a half-wave rectifying circuit arranged to produce a direct voltage varying substantially in proportion to the peak voltage amplitude of a selected polarity of said output voltage wave, each of said rectifying circuits being constituted of the gridcathode conduction paths of triodes respectively, and current indicating means connected between the cathodes of said triodes.

2. Apparatus for determining the position of a craft relative to two directive patterns of alternate energy transmissions, the durations of energy transmissions in a selected one of said patterns being longer than the duration of the alternate transmissions in the other of said patterns, comprising: radiant-energy receiving means for receiving energy transmitted in said two directive patterns and producing a detected output signal varyingin strength at the frequency of alternation of said patterns in accordance with the relative strength of energy received through said directive patterns, and a pair of opposed peak voltmeter circuits coupled to said receiving means to receive saidV detected output signal, the first of said peak voltmeter circuits being arranged to detect the positive peak strength and the second being arranged to detect the negative peak strength of alternating output voltage from said receiving means, said voltmeter circuits including triode electron discharge devices, and an indicating meter connected between the cathode leads of said triode devices, the difference of the positive and negative peak values of the alternating voltage providing an unambiguous measure of craft position relative to said two directive patterns.

3. Radio apparatus comprising radio receiving means including a detector; rst and second peak voltmeter circuits; alternating-voltage coupling means coupling said first voltmeter to said receiving means for energizing said first voltmeter with voltage peaks of a first polarity and coupling said second voltmeter to said receiving means for `energizing said second voltmeter with voltage peaks of the opposite polarity, plural triode means coupled to said first and second peak voltmeter circuits for comparing the detected values of peak voltages, and an indicating meter connected in the cathode leads of said triode means.

THOMAS M. FERRIIL, JR.

(References on following page) REFERENCES CITE) -The'fllowng referencesare'of recordl in the le of thispatent:

UNITED STATESV PATENTS Number YWeston Oct. `29, 1946 Number K v 2 ,418,284 2,424,560

Number Name Y Date Winchel Apr. 1, 1947 .Ear-p \Ju1yv29, 1947 yEarpei-,1511. Nov. 25, 1947 -`FOREIGN PATENTS 'Country Date Great'Brtain .=Jan. 17, 1936 ,'Fancev Dec.12, 1936 HFrance. `Dec. 16, 1940 yFrance :Apr. 22, 1940 .France Mar. 20,1944 

